Project summary Epilepsy is among the most common childhood neurologic disorders, affecting 40 children per 100,000 in the US alone. Children with seizure onset before age one have a six-fold increase in early mortality, due in part to a disproportionately poor response to conventional anticonvulsants. The availability of genetic testing has dramatically improved etiologic diagnoses of early-onset epilepsy, as ~26% of early-life epilepsy is now associated with pathogenic genetic mutations in a variety of genes such as ion channels. Yet, precisely- targeted therapeutic options remain limited. Missense pathogenic variants in KCNT1, a gene encoding a sodium-activated potassium channel, are causative for ~ 40% of cases of a severe infantile-onset epilepsy called epilepsy of infancy with migrating focal seizures (EIMFS), suggestive of a strong genotype-phenotype relationship. As a hallmark of EIMFS is medically-refractory seizures, targeting its pathogenic mechanism is an opportunity for novel anticonvulsant intervention. The goal of the proposed studies is to define the pathophysiologic mechanisms that lead to seizures in EIMFS so that anticonvulsant therapies can be rationally chosen and applied early in the disease course. Aim 1 delineates the cellular mechanisms governing a de novo KCNT1 gain-of-function variant in human neurons differentiated from patient-derived induced pluripotent stem cells (iPSCs). We hypothesize that altered KCNT1 channel kinetics result in increased persistent potassium current, impairing high-frequency firing of inhibitory neurons. Aim 2 combines detailed phenotyping of a mouse model of KCNT1-associated epilepsy with acute slice electrophysiology of labeled interneuron subpopulations. We hypothesize that hippocampal interneurons will be differentially affected by a gain-of- function Kcnt1 knock-in variant, evidenced by decreased action potential firing, with resultant decreased pre- synaptic GABA release and excessive excitatory neuron bursting. Taken together, these studies will broaden our understanding of the cellular mechanisms by which KCNT1 mutations contribute to the pathogenesis of severe childhood epilepsy, laying the groundwork for development of precise pharmacotherapies for EIMFS. This application is for a K08 Career Development Award for Tracy Gertler, M.D., Ph.D., Child Neurology Instructor at Lurie Children?s Hospital. To become an independent physician-scientist in the fields of ion channel physiology and neurogenetics, Dr. Gertler will commit the majority of her post-medical training to research in genetic epilepsy due to ion channelopathies. The division of pediatric neurology within the pediatrics department has an unwavering commitment to the career development of the candidate as she takes advantage of her neurophysiology background and adds training in applied stem cell biology and gene- editing and phenotyping of animal models of epilepsy under the mentorship of Drs. Alfred L. George, Jr. and Jennifer Kearney.